In hot gas filtration where high temperature dirty gas is passed through one or more porous filter elements to remove entrained solids from the gas, a layer of solid particles is deposited on the upstream surfaces of the filter elements as the gas is passed through the filter elements. It is necessary to remove such layers when the pressure across these layers unduly interferes with the operating efficiency of the filter. A preferred method of removing such layers is to apply bursts of a clean gas at high pressures, commonly referred to as jet pulses, to the downstream sides of the filter elements to cause a flow of cleaning gas in a direction opposite to the flow of gas through the filter elements during the normal filter cycle, thereby to blow the layer of solid particles off of the upstream surfaces of the filter elements and into the filter chamber. This can be done without taking the filter off line and without interrupting the flow of dirty gas to the filter elements by applying relatively high pressure bursts of cleaning gas to the downstream sides of the filter elements and staggering such bursts from one filter element or group of filter elements to another.
In order to prevent such high pressure bursts of cleaning gas from passing directly out of the filter without first passing in the reverse direction through the filter elements, it is known to position flow resistance devices, commonly known as enhancers, in the output lines from the filter elements. U.S. Pat. No. 4,909,813 describes such flow resistance devices in greater detail. These enhancers include a plurality of elongate, narrow passageways through which all of the cleaned gas must pass as it travels from the filter elements to the output of the filter and which provide a resistance to gas flow which is proportional to the velocity of the flow. Because of the relatively low velocity flow of gas through these passageways during the normal filter cycle, the enhancers do not provide any appreciable resistance to the normal flow of gas from the filter elements to the outlet of the filter. However, during the cleaning cycle when the velocity of gas to the downstream sides of the filter elements is appreciably increased by virtue of the high velocity jet pulses of cleaning gas, the enhancers provide a greatly increased resistance to gas flow which is greater than the resistance offered by the filter elements and the associated layers of particles deposited thereon to the reverse flow of gas therethrough. This increased resistance is sufficient to prevent any significant amount of cleaning gas from passing through the enhancers and thus out of the filter without first flowing back through the filter elements to dislodge the layers of solids from the upstream surfaces of the filter elements.
Because of the high temperature of the gas to which the enhancers are subjected, such enhancers have been made of ceramic, and for some applications, particularly where the holes through the enhancers must be extremely long and narrow to provide the required resistance during the backwash operation, manufacture of the enhancers has been both difficult and expensive. It would, therefore, be desirable to provide an improved enhancer which could be manufactured in a less expensive manner.
It would be further desirable to incorporate in such an enhancer a porous safety shield which is sufficiently strong to act as a safety backup filter to prevent dirty gas from passing through the filter in the event of a catastrophic break in one or more of the filter elements, or by a leak or failure of the filter element seals.